We intend to study the mechanism of late gene regulation in phage lambda. Lambda late genes are expressed under direction of the product of lambda regulatory gene Q, which is proposed to function as an anti-terminator. We will purify the product of lambda gene Q, using an assay that we have developed to measure late gene expression during DNA-dependent protein synthesis in vitro. We will attempt to demonstrate the regulatory function of the Q gene product in vitro using purified components of gene expression in order to understand the interaction by which it functions. We also will study the mechanism by which phage lambda repressor is inactivated during induction of a lysogen by ultraviolet light and other agents that damage DNA. The inactivation of lambda repressor is one of a set of cellular responses (termed the SOS functions) to such agents, including the activation of an error-prone repair mechanism that causes mutation. Since we can reproduce the inactivation of lambda repressor in vitro, we intend to purify the enzymatic machinery involved in order to determine the mechanism by which SOS functions are induced. Since proteolytic cleavage accompanies repressor inactivation, we will determine if this cleavage is the primary mechanism of inactivation.